Studies of the effects of neonatal infection with mouse thymic virus on the immune system showed that the virus has no effect upon B cells and has strikingly differential effects upon subpopulations of T cells; for example, cells producing direct GVH activity in thymus were depleted following infection with thymic virus whereas cells in thymus producing synergy when combined with lymph node cells were normal. Resistence to herpes virus infection in mice could be generated in mice with no demonstrable B cell function but intact T cell activity. This finding indicates that functional T cells and macrophages are sufficient to protect mice from a lethal challenge with this virus. A neutralizing factor for xenotropic murine leukemia virus was found in sera of congenitally athymic mice in titers equivalent to those found in phenotypically normal mice.